This invention relates to a viewing device, and more particularly to a wide angle viewing apparatus for security applications.
Door viewers for home security are well known. One common approach provides a peephole incorporating a miniature wide-angle lens. Peepholes suffer from the problem that the viewer's face must be pressed against a tiny hole.
U.S. Pat. No. 4,082,434 discloses a wide-angle door viewer comprising a concave objective lens, an intermediate concave lens and a convex eyepiece lens. The eyepiece lens is positioned at a predetermined distance from the objective lens. The intermediate lens corrects the aberration of the erect virtual image formed by the objective lens. The eyepiece lens magnifies the image formed by the intermediate lens. A magnified final erect virtual image is formed on the eyepiece lens. The U.S. Pat. No. 4,082,434 apparatus suffers from the problem that the location of the virtual image makes it impractical to insert a diffusing screen to provide a real image. Therefore, the user's eye must be positioned close to the eyepiece lens. Further, the small effective diameter of the concave objective lens results in a dim image. Increasing the effective diameter of the concave objective lens to provide a brighter image will allow visual access from outside unless a shutter is incorporated into the viewer.
U.S. Pat. No. 4,257,670 discloses an optical peephole device comprising three lens assemblies disposed serially along a common optical axis. The first assembly provides a doublet comprising a thick-edged meniscus and a double-concave lens. The second assembly comprises five identical plano-convex lenses equidistantly spaced from each other. The third assembly provides accommodation and comprises a plano-convex lens and an eyepiece. An erect virtual image formed by the meniscus is converted into an inverted real image by the plano-convex lens. The other plano-convex lenses correct aberrations and performs a second inversion on said inverted real image, such that the final erect real image is formed on the plano-convex lens. The disadvantage of the U.S. Pat. No. 4,257,670 apparatus is that although the image derived from the plano-convex lens is erect and real, the luminance of the final image suffers from the transmission losses incurred by the large number of lenses. As in the case of the U.S. Pat. No. 4,082,434 apparatus it is not possible to provide a real image and consequently the user's eye must be positioned close to the eyepiece. Furthermore, the device is not suitable for typical domestic door applications due to its large overall length.
U.S. Pat. No. 4,892,399 by Ohn discloses a door viewer comprising two prisms of rectangular isosceles triangle shape in cross section whose hypotenuse surfaces abut horizontally, a front convex lens, an intermediate plano-convex lens and a plano-convex eyepiece lens. The front convex lens has a front concave surface and a rear convex surface to correct chromatic aberration. The convex surfaces of the intermediate and eyepiece lenses are positioned face to face with each other to correct barrel distortion. The door viewer casts an image onto a ground glass screen formed on or provided abutting the eyepiece lens.
Door viewers based on the principles of the Ohn device are capable of providing a small real image, typically 25-60 millimeters in size, that can be viewed from a small distance. A commercially available door viewer based on the Ohn invention, known as the Ultra Vista door viewer, is distributed via the internet website www.doorviewers.ca. The Ultra Vista door viewer provides a 132° horizontal field of view and has an output image screen size of approximately 57 millimeters diameter. The image may be viewed from a range of approximately 2 meters and has the appearance of a miniature television display. The required door opening size is 56 millimeters for door thicknesses in the approximate range 20 to 45 millimeters. However, door viewers based on the Ohn invention suffer from the problem that the viewing screen size roughly determines the size of the door hole. It is therefore difficult to provide a large area screen using a viewer designed according to the principles of the Ohn invention.
There are several problems to be overcome in designing a door viewer with a small door aperture and a large area screen. To achieve a high image brightness the lens system requires a numerically low F-number, where F-number is defined as the focal length of the image projection lens divided by the effective aperture of the lens.
There are trade-offs to be made between the angle of surveillance, the range of screen viewing angles available to users, screen size and door size. Basic optical theory dictates that product of the entrance pupil area multiplied by the light collection solid angle corresponding to the field of surveillance should be roughly equal to the maximum screen viewing solid angle multiplied by the screen area. Providing a door viewer with a large viewing screen, a wide field of surveillance and a wide viewing angle will tend to increase the size of the entrance pupil. This in turn will increase the overall diameter of the lens and hence the size of door hole required.
In order to minimize the thickness of the door viewer the projection screen should have a large bend angle. In other words, the screen should be capable of directing light incident at a steep angle to the screen surface into an average direction substantially normal to the screen surface. It is difficult to maximize the photometric and screen thickness requirements simultaneously.
U.S. Pat. No. 6,511,186 by Burstyn et al discloses a screen in which light rays having acute incidence angles of a screen are deflected into the viewing space by Total Internal Reflection (TIR) Fresnel lens elements or by diffractive elements. However, the apparatus disclosed by Burstyn is not suitable for numerically small F-number illumination due to the small dimensions of the Fresnel lens facets.
There is a need for a low cost door viewer that offers a large viewable area, ideally around 100 to 150 millimeters diagonal. The field of view should be 130 degrees horizontal. The installation requirements should be no more demanding in terms of door alterations and installer skill than existing technologies. The screen should be viewable from a range of around 2 meters and for a representative range of viewer heights. Desirably, the door hole size should be in the range 40-60 mm. The device should have minimal projection from the front or rear surfaces of the door. The device should provide means for eliminating stray light that may impair the quality of the output image.
Thus there exists a need for an improved door viewer that can provide a wide field of surveillance, a large area viewable image and a thin form factor requiring only a small door aperture.